186 



MANUAL OF THE MOLLTJSCA. 



Animal. In the recent nautilus, the mandibles are horny, 

 but calcified to a considerable extent ; they are surrounded by 

 a circular fleshy lip, external to which are four groups of lahial 

 tentacles, twelve or thirteen in each group ; they appear to answer 



Fig. 50. Nautilus pompilius in its shell.* 



to the huccal memlrane of the calamary (Fig. 1). Beyond these, 

 on each side of the head, is a double series of arms, or brachial 

 tentacles, thirty-six in number ; the dorsal pair are expanded, 

 and united to form the hood, which closes the aperture of the 

 shell, except for a small space on each side, which is filled by 

 the second pair of arms. The tentacles are lamellated on their 

 inner surface, and are retractile within sheaths, or " digita- 

 tions," which correspond -to the eight ordinary arms c^ the 



* This woodcut and eighteen others illustrating the tetrabranchiata, are the property 

 of Dr. Gray, to whom we are indebted for their use. Fig. 50 represents the recent 

 nautilus, as it appears on the removal of part of the outer shell-wall (from the 

 specimen in the British Museum). The eye is seen in the centre, covered bj' the hood 

 (h) ; t, tentacles, nearly concealed in their sheaths ; /, funnel ; m, margin of the 

 mantle, very much contracted ; n, nidamental gland ; a, c, air-cells and siphuncle ; s, 

 portion of the shell ; a, shell-muscle. The internal organs are indicated by dotted 

 lines ; b, branchiae ; h, heart and renal glands ; c, crop ; g, gizzard ; /, liver ; o, ovary. 



